1. Field of the Invention:
This invention relates to an oxide crystal showing superconductivity at high temperatures and a production process thereof. More particularly, it relates to a superconductive oxide crystal, the superconductivity of which is hardly affected by a magnetic field, and a production process thereof
2. Description of the Prior Art:
Recently, oxides showing superconductivity at the temperature of liquid nitrogen have been found. However, almost all of these oxides are susceptible to magnetic fields so that the great problem in their practical use is that their critical temperature, i.e. the temperature at which where their electric resistance falls to zero, decreases when they are placed in a weak magnetic field Accordingly, a need continues to exist to produce a good superconductive oxide crystal having a high critical temperature and to elucidate the mechanism of its superconductivity in order that the commercial production of such oxide crystals may be realized at the earliest possible time.
In this connection, the May 11, 1987 issue of a journal, Nikkei New Material, at page 80, carries a report on a process for the production of a superconductive oxide single crystal. According to the report, a powder material, to be made into a superconductive product, is heated to above 1000.degree. C. in a platinum crucible to melt the material and then it is cooled slowly in order to let it grow into a single crystal. However, since the material is heated to above 1000.degree. C. in a platinum crucible, the decomposition reportedly takes place above 950.degree. C., and not only is the superconductivity of the product greatly reduced by contamination with platinum from the crucible, but, also the consumption of the precious metal is thought to bring about a great problem in commercial production. The report also points out that the critical temperature of the product is, in fact, very low.
As mentioned above, the fact is that a superior superconductive oxide crystal that has a high critical temperature and that is hardly affected by magnetic fields has not yet been produced when a conventional process is used.
Under the circumstances, the present inventors made intensive studies to eliminate drawbacks in conventional production processes and finally arrived at the present invention.